1. Field of the Invention
The invention is related to the field of computer systems, and in particular, to a modular bus bridge that can be readily converted from one version to another.
2. Statement of the Problem
Computer system block diagrams typically show a central processor unit (CPU) and an Input/Output (I/O) subsystem which includes peripheral devices such as disk drives, tape drives, and printers. For high-performance data transfers, I/O subsystem controllers physically interface to the CPU through one of several different internal bus architectures. The I/O controller normally plugs into the CPU bus and then cables to the actual peripheral devices through yet another bus called the peripheral bus. Popular peripheral bus interfaces include the Small Computer System Interface (SCSI) and Fiber Channel (FC). SCSI and FC protocols are specified by ANSI standards X3T9.2 and X3T9.3, respectively.
Conceptually, the SCSI I/O controller serves as a "bridge" between the CPU bus and the peripherals' SCSI bus in that it does wire transport layer conversions from the CPU bus to SCSI and also takes care of any software protocol translations and data buffering. The SCSI controller can also further fan out to multiple peripheral busses since the data bandwidth capabilities of the CPU bus significantly exceed that of the peripheral buses. The peripheral SCSI bus connects directly to the peripheral device (for example, a disk drive) within the CPU cabinet or it can be cabled remotely to another cabinet that contains the peripheral devices. When this remote bus interface leaves the CPU cabinet, it is then called the Host channel. Within the remote external cabinet a comparable bridge can be used to convert the host channel to fan out to additional peripheral busses, called Device channels. Thus, a bridge can be a one-to-many control device, for example, one SCSI Host channel to two SCSI Device channels, referred to as a "1.times.2 bridge".
Within the external cabinet the bridge can be designed as a single board (Motherboard) that plugs into a local backplane. All interfaces into the bridge including the Host and Device channels, and other supporting buses for management interfaces, for example RS-232, have to be accommodated in the design of the backplane. The backplane also needs to accommodate an interface to any visual displays through yet another interface. Design of the back plane and associated cabinetry are problematic and typically delay implementation of new bridges.
An alternative packaging scheme is to design the bridge in such a way that in addition to plugging directly to a backplane, it can also occupy an existing peripheral device slot (for example, 5.25 inch disk drive), in which case the Host and Device channels, and the management interfaces are cabled directly to the bridge. A simple I/O patch panel and display panel can also be designed that attach directly to the bridge.
Since current bridge products offer flexible host interfaces, the I/O patch panel could be designed to accommodate several different host channel technologies (for example, SCSI or FC) and the user selects the configuration merely by changing the cabling scheme; there is no need to change backplanes. This makes the customization and conversion effort between interfaces and bridge versions virtually transparent, and benefits the end-user in reduced implementation schedules, field upgrades, and re-installations. Resellers would benefit because they could easily modify the external bridge to fit end-users varied demands. Similarly, the display panel could be removed and easily replaced with a blank enclosure panel or just left off.